Scanning heat flow probe

ABSTRACT

A scanning heat flow probe for making quantitative measurements of heat flow through a device under test is provided. In one embodiment the scanning heat flow probe includes an electric current conductor in a cantilever beam connected to a probe tip and coupled to two voltmeter leads. The probe also includes two thermocouple junctions in the cantilever beam electrically isolated from the electric current conductor and the two voltmeter leads. Heat flow is derived quantitatively using only voltage and current measurements. In other forms, the invention relates to the calibration of scanning heat flow probes through a method involving interconnected probes, and relates to the minimization of heat flow measurement uncertainty by probe structure design practices.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention is directed to an apparatus for measuringheat flow and methods related to the fabrication and calibration of suchapparatus.

[0003] 2. Description of Related Art

[0004] One of the major difficulties in developing novel thin filmthermoelectric materials lies in obtaining consistent and accuratemeasurement of their thermal and electrical properties. Traditionalmethods cannot be easily extended to microscopic characterizationbecause of increased electrical and thermal parasitic losses associatedwith the probes used to perform the measurements. Additionally, the poorstructural stability of some of the novel materials being investigatedmakes using traditional probe methods unworkable.

[0005] For example, in the case of measurements using a probe, such asthe “ZT-meter,” the time-scales of the transients become short andintroduce errors in the electrical measurements. Scanning thermoelectricmicroscopy (STEM) based on atomic force microscope (AFM) probes arecapable of performing measurements of thermal and electrical propertiesof thermoelectric materials at these small scales. However, STEM basedon AFM probes still have several limitations. For example, presentprobes only give qualitative measurements of heat flow, which onlyallows one to determine whether there is more or less heat flow with onematerial versus another. Therefore, it would be desirable to have ascanning heat flow probe that allows quantitative measurements of heatflow to be made.

SUMMARY OF THE INVENTION

[0006] The present invention provides a scanning heat flow probe formaking quantitative measurements of heat flow through a device undertest. In one embodiment the scanning heat flow probe includes anelectric current conductor in a cantilever beam connected to a probe tipand coupled to two voltmeter leads. The probe also includes twothermocouple junctions in the cantilever beam electrically isolated fromthe electric current conductor and the two voltmeter leads. Heat flow isderived quantitatively using only voltage and current measurements. Inother forms, the invention relates to the calibration of scanning heatflow probes through a method involving interconnected probes, andrelates to the minimization of heat flow measurement uncertainty byprobe structure design practices.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The novel features believed characteristic of the invention areset forth in the appended claims. The invention itself, however, as wellas a preferred mode of use, further objectives and advantages thereof,will best be understood by reference to the following detaileddescription of an illustrative embodiment when read in conjunction withthe accompanying drawings, wherein:

[0008]FIG. 1 is an exemplary cross-sectional view of the scanningthermoelectric microscopy probe in accordance with the presentinvention;

[0009]FIG. 1A depicts an schematic diagram of an exploded view of thecomponents of the probe cantilever in accordance with the presentinvention;

[0010]FIG. 2 depicts a schematic diagram illustrating heat flow througha probe to a sample in accordance with the present invention;

[0011]FIG. 2A depicts a schematic diagram illustrating heat flow throughthe probe;

[0012] FIGS. 3A-3B depict schematic diagrams of the calibrationprocedure in accordance with the present invention;

[0013] FIGS. 4A-4N are exemplary cross sections illustrating a processof fabricating the scanning heat flow probe in accordance with thepresent invention;

[0014] FIGS. 5A-5N are exemplary top views illustrating a process offabricating the scanning heat flow probe in accordance with the presentinvention; and

[0015]FIG. 6 depicts an example of a graph of fractional heat flowuncertainty versus probe length in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] The present invention provides a method and apparatus formeasuring and characterizing the thermal and electrical properties ofthe materials. The invention as embodied herein makes use of temperatureand voltage sensors with a thermal probe to quantitatively measure heatflow through a tip of the probe. Also included is an electric conductorconnected to the probe tip to allow for electrical stimulation of thesample.

[0017] Referring now to FIG. 1, an diagram illustrating an exemplaryscanning heat flow probe 100 is depicted in accordance with the presentinvention. The probe shown in FIG. 1 is used to measure the thermalproperties of materials in a manner described in detail hereafter.

[0018] The probe 100 includes a probe body 150, a cantilever structure110, a first temperature sensing lead 106, a second temperature sensinglead 108, a first voltage sensing lead 101, a second voltage sensinglead 112, a current lead 160, a thermistor 118, and a probe tip 104. Aheater/cooler 116, such as a thermoelectric heater/cooler (TEC), mayalso be thermally coupled to probe 100 in order to control thebackground temperature of the probe 100. The leads 106, 108, and 114create two thermocouples at points 102 and 122 in cantilever 110 whichare used to calibrate probe 100 in a manner to be described hereafter,to permit quantitative measurement of the heat flow through the probetip 104 into sample 202 as in FIG. 2.

[0019] Referring to FIG. 1A, electrical current flows through probe 100via current conductor 160. Voltage readings are taken at points 102 and122 of current conductor 160 through leads 101 and 112. The currentconductor 160 is separated from the thermocouple leads 106, 108, and 114by an electrical insulator 190, such as, for example, silicon nitride.The distance between the proximal thermocouple at 122 and the distalthermocouple at 102 is typically between 100 and 900 microns. The distalthermocouple at 102 measures the temperature near the tip 104 usingleads 108 and 114. A differential thermocouple formed from the proximalthermocouple at 122, and the distal thermocouple at 102 using leads 108and 106, measures the temperature drop along the beam. In oneembodiment, cantilever 110 has a width of approximately 8 microns. Inone embodiment, the probe tip 104 may be constructed from tungsten. ATEC heater/cooler 116 may be thermally coupled to the scanning heat flowprobe 100 to bias the temperature at 122. By varying the temperature at122, the heat flow through the probe can be adjusted to a desired value,including zero.

[0020] In some preferred embodiments, the probe includes a radiationshield surrounding but not contacting the probe cantilever 110 frompoints 122 to 102 to mitigate radiation effects. Also, the probe andsample should be used in vacuum to mitigate gaseousconvection/conduction effects.

[0021] While the probe structure shown in FIG. 1 shows a sharp probetip, the probe tip may be of any shape desirable. For example, thecone-shaped probe tip may be very narrow or very wide in diameter, mayhave any value interior angle at the tip, and the like.

[0022] Calibration

[0023] In order to measure heat flow accurately, the probe 100 must becalibrated. In designing and calibrating the probe 100, one goal is theminimization of error in the probe in order for the probe to provideaccurate measurements of heat flow in a sample. In order to do this, theerror in the heat flow, Q, through the probe should be minimized as muchas possible.

[0024] As depicted in FIG. 2A, the heat flow through the probecantilever 110 may be characterized by two components: a component,Q_(j), due to joule heating from current flowing through the electricalconductor 160 and a component, Q_(m), due to the temperature gradientacross the probe cantilever 110 between points 122 and 102.

[0025] The joule heating component, Q_(j), may be measured from thevoltage across the current conductor 160 with leads 101 and 112, and ameasurement of the current through the current conductor 160. Forpurposes of calibration, it is accepted that one half of the heat for auniform structure, Q_(j), generated by joule heating flows toward thedistal end of the probe 100 and one half flows toward the proximal endof the probe 100.

[0026] Thus, heat flow, Q, through the probe 100 into the sample isgiven by the following relation:

Q=Q _(m)+½Q _(j).

[0027] Since Q_(m) is related to the temperature gradient across theprobe, ΔT, and to the thermal resistivity, R_(th), by the followingequation:

Q _(m) +ΔT/R _(th),

[0028] and since the heat flow due to joule heating is given by therelationships described above, the heat flow Q is given by the followingrelationship:

Q=ΔT/R _(th)+½IV _(e)

[0029] where V_(e) is the voltage across the current conductor 160between points 122 and 102 in the cantilever 110. The temperature acrossgradient may be expressed as Vth/a where V_(th) is the differencebetween the voltage across the distal thermocouple (V_(TCd)) and thevoltage across the proximal thermocouple (V_(TCP)) and a is the Seebeckcoefficient across the junction of the differential thermocouple. Thus,Q may be represented by the following relationship:

Q=V _(th)/(aR _(th))+½IV _(e)

[0030] Therefore, the uncertainty, σQ, in the heat flow may be expressedas:

(σQ)² =Q _(m) ²[(σV _(th) /V _(th))²+(σaR _(th) /aR _(th))²]+(½Q_(j))^(2[(σ) V _(e) /V _(e))²+(σI/I)²]

[0031] This introduces four error terms: the error in the thermocouplevoltage measurement, σV_(th), the error in the calibration of aR_(th),σaR_(th), the error in the current conductor voltage measurement σV_(e),and the error in the measurement of the current through the currentconductor, σI. However, three of these terms are determinable from thetolerances provided with the commercially available instruments utilizedto make the voltage and current measurements. This leaves only theuncertainty in αR_(th) to be determined.

[0032] In order to determine the uncertainty in αR_(th), the calibrationmethod of the present invention utilizes two scanning heat flow probesas depicted schematically in FIGS. 3A-3B. In FIG. 3A, two scanning heatflow probes 301 and 302 are oriented such that their tips touch. Theprobes are shown oriented linearly in FIGS. 3A-3B, however, theparticular orientation between the two probes is not important. In step1, a temperature gradient ΔT is created across the two probes by, forexample, heating one probe with a heater. The temperature gradientinduces a heat flow, Q, from one probe 301 to the other probe 302 asdepicted in FIG. 3A. The voltages V_(tha1) and V_(thb1) of the twodifferential thermocouples in probes 301 and 302 are measured. Since theheat flow through the two probes is equal:

αR _(tha) /αR _(thb) =V _(tha1) /V _(thb1).

[0033] The second step is depicted in FIG. 3B. In FIGS. 3B, the same twoprobes are used though they need not be in the exact same configuration.A current, I, is passed from probe 301 into probe 302 with voltagemeasurements made at points 310-313 using voltage leads 101 and 112 ofeach probe 301, 302. It is assumed that one half of the heat produced byjoule heating in probe 301 passes into probe 302 and that one half ofthe heat produced by joule heating in probe 302 passes into probe 301.There is also joule heating produced by the current I passing from probe301 to probe 302 in the region between points 311 and 312. However,since this section includes portions of both probes, no assumptions aremade about how much of this heat passes through probe 301 and how muchpasses through probe 302. Therefore, a fraction, fQ_(c), of the heat,Q_(c), produced by joule heating in region c passes into probe 302 andthe remaining fraction of Qc (i.e. (1−f)Q_(c)) passes into probe 301.Furthermore, since the current flowing through the two probes is equal,the following relation may be determined:

V _(thb2) /αR _(thb)+(−V _(tha2) /αR _(tha))=Q _(c)+½Q _(a)+½Q _(b)

[0034] Therefore, since there are now two equations, aR_(th) can beisolated for one of the probes. Thus, the following equation may beobtained for probe a:

αR _(tha)=((V _(tha1) /V _(thb1))V _(thb2) −V _(tha2))/(I(V _(c)+½V_(a)+½V _(b))

[0035] Thus, the uncertainty in aR_(tha) (i.e. σαR_(tha)) depends onlyupon measurable quantities (e.g., I and V). Thus, the uncertainty in theheat flow through the probe depends entirely on measurable quantities.

[0036] Probe Formation Process

[0037] FIGS. 4A-4N are exemplary cross sections illustrating a processof fabricating the scanning heat flow probe 100 and FIGS. 5A-5N areexemplary corresponding top views in the process of fabricating thescanning heat flow probe 100. The scanning heat flow probe 100 iscomprised of a number of different layers of material. The particularmaterials described hereafter with reference to the exemplary embodimentare meant to be for illustrative purposes and other materials havingsimilar properties may be used in replacement or in addition to thematerials described herein without departing from the spirit and scopeof the present invention.

[0038] The formation of the scanning heat flow probe 100 will now bedescribed with reference to FIGS. 4A-4N and 5A-5N. The mechanisms usedto create the various layers of the probe, such as deposition andetching, are generally known in the art of semiconductor chipmanufacture. However, these mechanisms have not previously been used tocreate the structure herein described.

[0039] The process starts with a silicon wafer 402 sandwiched betweentwo polished silicon nitride layers 404 and 406. The topside siliconnitride layer 404 which will in part form beam 191 (FIG. 1A) isselectively etched, for example by a reactive ion etch (RIE), to removea portion of the silicon nitride as depicted in FIG. 4B for front-backalignment marks 460. For ease of illustration, the alignment marks 460are not depicted in the next few sets of diagrams.

[0040] Next, using two separate masks, nickel 410 and chromium 408 leadsare deposited onto silicon nitride 404 as depicted in FIGS. 4C and 5C.Preferably, the chromium leads 408 are approximately 60 nm thick and thenickel 410 leads are approximately 80 nm thick over 3 nm of chromium. Alayer of silicon nitride 412 is then deposited over the chromium 408 andnickel 410 layers as well as over the remaining original silicon nitridelayer 404 as depicted in FIGS. 4D and 5D by, for example, a PECVDprocess.

[0041] The resulting structure is then masked and a reactive ion etch isused to pattern the silicon nitride cantilever as depicted in FIGS. 4Eand 5E. Next a tungsten (W) layer 414 is sputtered onto the exposedsurface of the structure, preferably to a thickness of 2 micronsresulting in a structure as depicted in FIGS. 4F and 5F. Next a layer ofthick photoresist 416, such as, for example, AZ or Shipley, is formedover the structure by spinning resulting in the structure as depicted inFIGS. 4G and 5G. Preferably the photoresist 416 is spun to a thicknessof 3 microns. The photoresist 416 is then patterned into the tip shapeas depicted in FIGS. 4H and 5H.

[0042] Next, a wet etch is performed on the W layer 414 undercutting thephotoresist pattern 416 resulting in a structure as depicted in FIGS. 4Iand 5I. Next, the photoresist 416 is stripped resulting in a structureas depicted in FIGS. 4J and 5J. The structure is then masked and thePECVD silicon nitride 412 is etched over the nickel and chromiumcontacts 418, 420 resulting in the structure depicted in FIGS. 4K and5K. The structure is then masked again and the gold contacts 422 and 424are formed to the tungsten, nickel, chromium and resistor lines 426-428as depicted in FIGS. 4L and 5L. Next, another mask is utilized topattern the backside silicon nitride 406. RIE results in the structureas depicted in FIGS. 4M and 5M. After a final bulk silicon etch in KOH,the final resulting structure is formed as depicted in FIGS. 4N and 5N.

[0043] The probe created using the process described above can be usedfor making measurements in many different applications. The probe may beused to measure thermoelectric properties of nano-scale structures,profiling of silicon dopants of semiconductor materials, characterizinggiant magneto-resistive heads, and the like. The present invention isnot limited to any one application of the probe and is intended to coverall possible applications to which the probe may be made. Those ofordinary skill in the art will appreciate that the probe of the presentinvention is preferably utilized along with a computing system in whichthe calibration and computations described above and hereafter areperformed. The probe is used to provide measured quantities which arethen processed by the computing system to calibrate the probe andgenerate values for the heat flow properties of the materials undertest.

[0044] Method for Optimizing Design of Probe

[0045] In designing the probe, a method for determining a substantiallyoptimum design for the probe in accordance with the present inventionmay be used. This method determines, for example, the optimum length ofthe probe cantilever, given the operational parameters under which theprobe is intended to be used as well as the values of other parametersalready chosen. This method of the current invention is a method forexploring the design space of a scanning heat flow probe and selectingthe design parameters that minimize the errors in probe measurement,specifically the uncertainty in heat flow through the probe.

[0046] Method Overview:

[0047] An equation describing the heat, Q, flowing through the probe isformed. This equation is based upon

[0048] 1 Design parameters (values, no uncertainties)

[0049] 1 material properties/composition

[0050] 1 electrical resistance

[0051] 2 thermal conductivity

[0052] 3 Seebeck coefficient

[0053] 2 Geometries

[0054] 1 Beam width/length

[0055] 2 Metal width/thickness

[0056] 3 Thermocouple width/thickness

[0057] 3 Configuration

[0058] 1 Implied in the thermal and electrical networks

[0059] 2 Measurement environment (values, no uncertainties)

[0060] 1 Heat flow, Q, through the probe

[0061] 2 Current through the probe

[0062] 3 Calibration conditions (values, no uncertainty)

[0063] 1 Two step/two probe method

[0064] 2 Calibration temperature difference

[0065] 3 Calibration current

[0066] 4 Contact resistance

[0067] 4 Measurement equipment (uncertainties)

[0068] 1 Manufacturer stated uncertainty in the measurement for thegiven operational point

[0069] All the stated values, while not exact to the usage condition,are to place the probe in a certain operational range. The sole sourceof uncertainty is from the measurement equipment. Variation in theoperational conditions of the probe will have an effect on the optimalprobe design parameters, but this is a second order effect. Largedeviations in operating conditions will substantially change the optimaldesign parameters and lead to degraded performance. This leads to theuse of different probes for different operational conditions, just as amulti-meter changes ranges to improve performance.

[0070] Any probe will not be used in the exact operational range thatdetermined the probes design parameters, but this is not cause for theperformance of the probe to become suspect. When an actual measurementis made, the instrument measurements are entered into the heat flowequation yielding a heat flow based on measured values, no designparameters or constants. The calibration parameters are derived frommeasured values so they do not invalidate the previous statement. Theuncertainty of the heat flow is also based on measured values and themanufacturers stated uncertainties of the measurement equipment.

[0071] So probe design and use come in two phases. In the design phaseevery parameter is chosen or derived from chosen values. The designparameters are varied, namely length, and the design parameters arechosen such that error in the measurement of the heat flow is minimized.In the second phase all design parameters are forgotten, and the heatflow and uncertainty are based on measured values and the manufacturersstated uncertainty for the equipment.

[0072] Equations:

[0073] The following list of equations may be simplified to a singleequation expressing the uncertainty on the measurement of the heat flowthrough the probe. Any parameter can be varied, though length of theprobe is most common, and a curve of uncertainty vs. that parameter canbe formed. From this curve an optimal range for that design parametercan be chosen. An example of a graph of length of probe versusuncertainty in accordance with the present invention is depicted in FIG.6. Good results for probe performance may be obtained using a cantileverbeam having a length falling in region B. The increase in uncertaintyfor lengths smaller than this in region A is due to instrumentationuncertainty since such lengths result in smaller temperature gradientsthereby becoming almost unmeasurable by the instrumentation. Theincrease in uncertainty for lengths in region C result from increasejoule heating because of the increased length of the cantilever beam.

[0074] While this method is not sophisticated like a global errorminimization it turns out that most of the parameters should bemaximized or minimized individually and there is no system level tradeoff. For the length of the beam there is a system level trade off.

[0075] A longer beam produces a large temperature drop for a given heatflow making the thermocouple voltage easier to measure. This is true upto a point. Over some range the voltmeter has a fairly flat % errorversus voltage. Increasing the voltage no longer decrease theuncertainty in heat flow.

[0076] A longer beam also has more resistance in the current conductor.This causes more heat to be generated. When the heat generated in thecurrent conductor is greater than the heat that should be delivered tothe object under test, the probe must have a counter heat flow (themeasured heat flow) that reduces the heat flow to the object. Thissubtraction of two large values causes a large uncertainty on theresult. It is for this reason that the beam length has an optimal valuefor a given operation condition.

[0077] In the equations that follow, a couple functions are used. Thefunction Xspec gives the uncertainty in a measurement for a giveninstrument and measured value. The function mat_table gives therequested material property for a given substance. L = 900e−6 [m]distance between 122 and 102 I = 100e−6 [A] current through 160 sI =Xspec(I, current source); uncertainty in the current above Q = 2e−6 [W]Heat flow through the probe a = 30e−6 [V/K] Seeback coefficient of thethermocouple material pair SiNx_width = 8e−6 [m] beam 191 widthSiNx_thick = 1.9e−6 beam 191 thickness [m] ni_width = 1e−6 [m] Nickellead 114 of thermocouple ni_thick = 80e−9 [m] cr_width = 1e−6 [m]Chromium leads 106 & 108 of thermocouple cr_thick = 60e−9 [m] I_width =2e−6 [m] Current conductor 160 I_thick = 200e−9 [m] V_width = 1e−6 [m]Voltage sense wires 110 & 112 V_thick = I_thick; pho_e =mat_table(‘W’,‘electrical_resistivity’); Current conductor Re =pho_e.*L./(V_width.*V_thick); Ve = I.*Re; sVe = Xspec(Ve, voltmeter);pho_(‘3)th = mat_table(‘W’,‘thermal_conductivity’); Rth_SiNx =L./(mat_table(‘SiNx’,‘thermal_conductivity’).*SiNx_w idth.*SiNx_thick);Rth_tc = L./(... mat_table(‘Ni’,‘thermal_conductivity’).*ni_width.*ni_thick +... mat_table(‘Cr’,‘thermal_conductivity’).*cr_width.*cr _thick); Rth_Vsense = L./(pho_th.*V_width.*V_thick); Rth_Wire =L./(pho_th.*I_width.*I_thick); Rth = L./(...mat_table(‘SiNx’,‘thermal_conductivity’).*SiNx_width .*SiNx_thick +...mat_table(‘Ni’,‘thermal_conductivity’).*ni_width.*ni _thick +...mat_table(‘Cr’,‘thermal_conductivity’).*cr_width.*cr _thick +...pho_th.*V_width.*V_thick +... pho_th.*I_width.*I_thick); Rthp =1./(1./Rth_SiNx + 1./Rth_tc + 1./Rth_Vsense + 1./Rth_Wire); Vth =a.*(Q−1/2*I.{circumflex over ( )}2.*Re).*Rth; sVth = Xspec(Vth,voltmeter); %Calibration portion aRth = a.*Rth; Ical =100e−6*ones(n_X,n_Y); sIcal = Xspec(Ical, current source); dV1 =Re.*Ical; sdV1 = Xspec(dV1, voltmeter); dV2 = Re.*Ical; sdV2 =Xspec(dV2, voltmeter); Rcontact = 100 [Ohms] dVc = Ical.*Rcontact; sdVc= Xspec(dVc, voltmeter); sW = W.*sqrt(... (sI./I).{circumflex over( )}2 + (sdVc./dVc).{circumflex over ( )}2 ); Qcal =Ical.*(dVc+0.5*(dV1 + dV2)); sQcal = Ical.*sqrt(...(Qcal.*sIcal./Ical).{circumflex over ( )}2 + ... (sdVc).{circumflex over( )}2 + ... (sdV1/2).{circumflex over ( )}2 + ... (sdV2/2).{circumflexover ( )}2 ); QdTcal = 2e−6 [W] VthlT = aRth.QdTcal; sVthlT =Xspec(VthlT, voltmeter); Vth2T = aRth.*QdTcal; sVth2T = Xspec(Vth2T,voltmeter); Vth1I = aRth.*Qcal/2; sVth1I = Xspec(Vth1I, voltmeter);Vth2I = aRth.*Qcal/2; sVth2I = Xspec(Vth2I, voltmeter); aRthp =(Vth1T.*Vth2I./Vth2T + Vth1I(./Qcal; %all together saRth = sqrt(...(Vth2I.*sVth1T./(Qcal.*Vth2T)).{circumflex over ( )}2 + ...(Vth1T.*Vth2I.*sVth2T./(Qcal.*Vth2T.{circumflex over ( )}2)).{circumflexover ( )}2 + ... (Vth1T.*sVth2I./(Qcal.*Vth2T)).{circumflex over ( )}2 +... (sVth1I.*Vth2I./Vth2T + Vth1I).*sQcal./Qcal.{circumflex over( )}>2).{circumflex over ( )}2 ); %End Calibration portion Qj =I.{circumflex over ( )}2.*Re; Qm = Q − 1/2*Qj; sQ = sqrt(...(Qm).{circumflex over ( )}2.*( (sVth./Vth).{circumflex over ( )}2 +(saRth./aRth).{circumflex over ( )}2 ) + ... (1/2*Qj).{circumflex over( )}2.*( (sVe./Ve).{circumflex over ( )}2 + (sI./I).{circumflex over( )}2 ) );

[0078] The description of the present invention has been presented forpurposes of illustration and description, and is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art. The embodiment was chosen and described in order to bestexplain the principles of the invention, the practical application, andto enable others of ordinary skill in the art to understand theinvention for various embodiments with various modifications as aresuited to the particular use contemplated.

What is claimed is:
 1. A scanning heat flow probe, comprising: anelectric current conductor in a cantilever structure connecting a probetip at one end and to a probe body at an opposite end; first and secondvoltage sense leads coupled to the electric conductor at first andsecond sense points; and two thermocouple junctions in the cantileverstructure and operatively positioned proximate the first and secondsense points.
 2. The scanning heat flow probe as recited in claim 1,further comprising: a dielectric layer situated between the electriccurrent conductor and the two thermocouple junctions in the cantileverstructure.
 3. The scanning heat flow probe as recited in claim 2,wherein the dielectric layer comprises silicon nitride.
 4. The scanningheat flow probe as recited in claim 2, wherein the cantilever structure,the electric current conductor, the voltage sense leads, the dielectriclayer, and the two thermocouple junctions define a heat flow path to theprobe tip.
 5. The scanning heat flow probe as recited in claim 1,wherein the two thermocouple junctions are coupled to first, second, andthird temperature responsive sense leads.
 6. The scanning heat flowprobe as recited in claim 1, wherein the two thermocouple junctionscomprise nickel and chromium connections.
 7. The scanning heat flowprobe as recited in claim 1, wherein the first and second voltage senselead couplings provide a measure of voltage drop along the electricalcurrent conductor between first and second sense points.
 8. A scanningheat flow probe, comprising: a probe body; a probe tip; a cantileverstructure connecting the probe body to the probe tip; a electric currentlead in the cantilever structure coupled to the probe tip and to theprobe body with voltage measurement leads coupled to the electriccurrent lead at a proximal point and a distal point; a firstthermocouple junction positioned in the cantilever at the proximal pointand electrically isolated from the electric current lead and the voltagemeasurement leads; and a second thermocouple junction positioned in thecantilever at the distal point and electrically isolated from theelectric current lead and the voltage measurement leads; wherein thethermocouple junction responses at the proximate and distal points,voltages measured at the proximal and distal points, and currentmeasured through the electric current lead are related to the heat flowproperties of the cantilever structure.
 9. A method of calibrating ascanning heat flow probe, the method comprising: placing the distal endsof two scanning heat flow probes having proximate and distal ends ofcantilever structures in contact with each other; establishing atemperature gradient between the proximate ends of the two probes;measuring thermocouple junction voltages in each of the two probes;establishing a current through the two probes; measuring voltage dropsin each of the two probes; and deriving a product of thermocoupleSeebeck coefficient and cantilever structure thermal resistance from themeasured voltages.
 10. The method as recited in claim 9, wherein thethermocouple junction voltages correspond to temperature drops througheach of the probes.
 11. The method as recited in claim 9, wherein thevoltage drops corresponding to the Joule heating in each of the probes.12. The method as recited in claim 9, wherein said deriving step relatesvoltage drops to heat flow.
 13. A method for fabricating a scanning heatflow probe, the method comprising: forming thermocouple junctions on adielectric structure; forming a dielectric insulator over thethermocouple junctions; forming an electric current conductor withvoltage sense leads proximate the thermocouple junctions over thedielectric insulator; and selectively removing material from the layersto form a cantilever structure.
 14. The method as recited in claim 13,further comprising: forming a probe tip at one end of the electriccurrent conductor which is in electrical contact with the electriccurrent conductor.
 15. The method as recited in claim 14, wherein theprobe tip comprises tungsten.
 16. The method as recited in claim 13,wherein the thermocouple junctions comprise nickel and chromium.
 17. Themethod as recited in claim 16, wherein the nickel is formed to athickness of approximately 80 nanometers.
 18. The method as recited inclaim 16, wherein the chromium is formed to a thickness of approximately60 nanometers.
 19. The method as recited in claim 13, wherein thedielectric structure comprises silicon nitride.
 20. The method asrecited in claim 13, wherein the dielectric insulator comprises siliconnitride.
 21. A method for optimizing a design parameter for a scanningheat flow probe, the method comprising: determining values for aplurality of known design parameters; determining measurement deviceuncertainties for a plurality of measurement devices; determining arelationship between an unknown design parameter and the heat flowuncertainty based on the known design parameters and the measurementdevice uncertainties; and determining a value for the unknown designparameter that substantially minimizes the probe uncertainty.
 22. Themethod as recited in claim 21, wherein determining a relationshipbetween the unknown design parameter and the heat flow uncertaintycomprises creating a graph plotting a curve of the values of heat flowuncertainty versus corresponding values of the unknown parameter. 23.The method as recited in claim 21, wherein the unknown design parameteris the length of a cantilever beam structure.